Description of the Prior Art
In a system of synchronous communication between data processing equipments one of the most widely used methods for sending messages consists in defining a frame structure characterized by:
a synchronization flag, PA1 special coding of the data to be transmitted so that the synchronization flag cannot be recognized in the middle of the data stream transmitted. PA1 if a first subchannel is identified the second subchannel corresponds to a second subframe, PA1 if a half-rate frame is identified, recognition of the synchronization pattern in the second subchannel implies the presence of a half-rate frame.
One of the best known ways to implement this method is to choose a flag which is a constant stream of P binary zeros followed by a binary 1. The data is then coded simply by inserting a binary 1 each time that a series of (P-1) binary 0 has been transmitted. For example, if the flag is `00001`, the message `0010 0000 10` is transmitted in the form: `00001 0010 00100 10`. The six underlined digits represent the synchronization flag and the inserted binary 1 (the spaces are included only to facilitate reading).
This method has a drawback: the time to transmit a message depends on its contents, which is a serious problem if a fixed routing time is required.
The known solution to this problem is to insert a binary 1 every (P-1) data bits transmitted: it is then certain that P consecutive binary zeros will never be encountered and the transmission time is always the same, regardless of the data transmitted. A well-known example of this method is the use of V.110 frames as defined by the CCITT (Comite Consultatif International du Telephone et du Telegraphe). These frames comprise a flag made up of eight binary zeros followed by a binary 1, a binary 1 being then inserted every seven bits to form a frame of 80 bits, 17 bits used for synchronization and 63 bits for the data.
A frame can be represented by a table with P columns and L rows. The first row, known as the locking row, includes P binary zeros and subsequent rows, known as data rows, each comprise a synchronization bit at binary 1 followed by (P-1) data bits.
When data is transmitted by means of such frames the transmission channel often has a data signaling rate which is greater than that required to convey the frames produced by an equipment.
To economize on transmission channels it is natural to use multiplexing whereby the channels are divided into subchannels so that the data signaling rate of one subchannel conveys the data from one equipment.
For example, a full-rate channel comprises two half-rate subchannels, the bits of the channel being bits from each of the two subchannels alternately. Thus the odd-numbered bits of the channel are from a first subchannel and the even-numbered bits of the channel are from a second subchannel.
This use of a channel to convey two subchannels is feasible but it rules out the interchangeable use of two half-rate subchannels or one full-rate channel because the simultaneous reception of two subchannels can be interpreted as the reception of a single full-rate channel.
If a full-rate frame of P columns and L rows and a half-rate frame of P' columns and L' rows are chosen, with an even value for P an advantageous solution is to take P'=P/2 and L'=L.
The full-rate frame can be divided into first and second subframes respectively comprising the odd-numbered bits and the even-numbered bits of the full-rate frame. The first subframe is identical to the half-rate frame and the second subframe begins with a locking row of P' consecutive binary 0 followed by P'.multidot.(L-1) data bits.
Given that the first subframe is identical to the half-rate frame and that the locking row of P' consecutive binary 0 can appear in the second subframe or in the half-rate frame, some means of discrimination is required.
One simple means of discrimination is to provide an identification field which is substituted for data bits and indicates either a subframe or a half-rate frame. For example, a header bit can be replaced with a binary 0 in the subframe so that the receiver detects a synchronization error in the full-rate frame. The receiver must always receive a binary 1 at the start of a data row. However, if the header bit is forced to binary 0, binary 1 bits must be inserted on either side of this binary 0 bit to prevent the occurrence of P' consecutive binary 0.
A synchronization pattern is then defined as the combination of the synchronization flag and the identification field.
The synchronization pattern can occur in a half-rate frame or in the second subframe of a full-rate frame, since the latter includes a sequence of P'.multidot.(L-1) indeterminate bits.
It is assumed that the first and second subframes of a half-rate frame are conveyed by respective first and second subchannels.
It is clear that it is not possible to tell whether the subchannel is conveying a half-rate frame or the second subframe of a full-rate frame by analyzing the subchannel only. The data specifying the nature of the second subchannel is available only in the first subchannel. It is therefore necessary to wait for the processing to identify the first subchannel:
Thus if the second subchannel conveys a half-rate frame, processing of this frame is deferred until the end of the identification processing. This introduces an undesirable time-delay into the transmission system.
Although the processing of the first subchannel is independent, the processing of the second subchannel is conditioned by the processing of the first subchannel. It follows that the modules processing each subchannel must communicate with each other, which complicates the implementation of the receiver.
An object of the present invention is therefore a frame structure defining a full-rate frame and a halfrate frame and a transmitter and a receiver between which such frames are exchanged and which can process a halfrate frame without any time-delay.